Subsequent events (Details) - Subsequent Events [Member] $ / shares in Units, ft³ / d in Millions, $ in Millions |
Oct. 31, 2018
USD ($)
ft³ / d
kV
TransmissionLine
$ / shares
|
|---|---|
| Dividends approved [Abstract] | |
| Dividend approved (in dollars per share) | $ / shares | $ 0.36 |
| Dividend approved expected date to be paid | Dec. 14, 2018 |
| Dividend approved record date | Nov. 30, 2018 |
| Power Substation [Member] | |
| Business acquisition [Abstract] | |
| Gross capacity | kV | 220 |
| Number of transmission lines acquired | TransmissionLine | 2 |
| Period of contract | 15 years |
| Pemex Compression System [Member] | |
| Business acquisition [Abstract] | |
| Standard installed gas compression capacity of facility per day | ft³ / d | 450 |
| Ownership percentage acquired | 5.00% |
| Additional ownership percentage to be acquired | 65.00% |
| Remaining ownership percentage to be acquired one year after COD | 30.00% |
| Total estimated equity investment | $ | $ 150 |